1. Field of the Invention
The present invention generally relates to a method of curing a coating composition including a radical polymerizable compound and an organoborane-amine complex. More specifically, the invention relates to a method of forming in-situ a decomplexing agent for decomplexing the organoborane-amine complex and initiating polymerization of the radical polymerizable compound.
2. Description of the Related Art
Automotive finishing and refinishing is a growing industry in the United States and other countries due to an ever increasing number of vehicles being produced. Typically, original equipment manufacturing (OEM) automotive finish coatings and aftermarket refinish automotive coatings are formed from one-part or two-part compositions that require physical mixing of two condensed phases. This mixing typically limits control of curing, increases cure times, and makes application of the coating compositions to complex shapes and sequestered surfaces difficult.
As such, an interest in using quick cure technology, such as UV-cure and electron beam technology, has arisen in automotive finish and refinish coatings. These technologies utilize free electrons, whether as radicals formed by UV light or as electrons formed from electron beams, to polymerize and cure the coating compositions. These technologies minimize the curing times of the coating compositions as compared to differently cured solvent-based and water-based coating compositions but require expensive equipment such as UV lamps, vacuums, filaments, etc. Thus, use of these technologies is expensive, time consuming, and labor intensive, and may result in film shrinkage and oxygen surface inhibition.
Finish and refinish coatings requiring heat to cure typically employ use of large ovens that can accommodate entire automobiles or components thereof to initiate crosslinking. Typically, the coating compositions are applied to automobile components, which are then passed through the ovens to cure the coating compositions and thereby form the cured coatings. However, use of the ovens is very energy intensive, expensive, and has an adverse impact on the environment. In OEM automobile production facilities, the ovens occupy large footprints and are cumbersome to use.
Other technology has also been developed to improve the speed and efficiency with which the coating compositions are cured. This technology utilizes boron compounds, e.g. organoborane initiators, to form radicals that polymerize organic monomers and cure the coating compositions. Organoborane initiators initiate free radical polymerization in the coating compositions and promote adhesion of the resulting cured coatings to low surface energy substrates due to the ability of the organoborane initiators to generate radicals, which polymerize the organic monomers. Without intending to be bound by any particular theory, it is believed that diffusion limited oxidation of the organoborane initiators, and production of the radicals therefrom, is driven by the thermodynamic stability of boron-oxygen bonds in the organoborane initiator and causes the organoborane initiators to be pyrophoric in oxygen. Due to this reactivity, it is known to stabilize the organoborane initiators with blocking agents that render the organoborane initiators less susceptible to oxygen insertion and premature radical generation. The blocking agents are separated from the organoborane initiators under controlled conditions (e.g. with the application of heat or through exposure to a decomplexing agent) to release the organoborane initiators and initiate free radical formation via reaction with oxygen.
Even using the aforementioned technologies, coating compositions applied to complex shapes and surfaces or on multi-component parts are typically not able to cure effectively due to an inability of UV light to reach all portions of the coating compositions. Alternatively, some coating compositions cannot be exposed to UV light without suffering damage and thus also have a tendency to cure ineffectively. Still further, curing through use of UV light can be inhibited by oxygen at a surface of the applied coating composition. Such oxygen surface inhibition typically results in incomplete curing of the coating composition, leading to cured coatings that are tacky or lack scratch resistance.
As a result, technology has also been developed to alleviate oxygen surface inhibition. This technology includes use of gaseous atmospheres with UV light and irradiation of coating compositions in gaseous atmospheres. The gaseous atmospheres limit amounts of oxygen that are present during curing, thereby limiting oxygen surface inhibition. However, UV and radiation sources used in this type technology are typically disposed at great distances from the coating compositions such that incomplete curing is reduced but not eliminated. Since radiation sources typically emit large amounts of heat, it is difficult to bring the radiation sources within the gaseous atmospheres and shorten the distances between the radiation sources and the coating compositions to be cured. The heat from the radiation sources causes strong vortexing in the gaseous atmospheres and contaminates the atmospheres with oxygen, thus negating the benefits of using this technology.
One derivative of this technology utilizes organoborane-amine complexes and gaseous initiating agents to effect curing. As is known in the art, and as alluded to above, blocking agents may be separated or decomplexed from the organoborane initiators in the organoborane complex through exposure to a decomplexing agent to release the organoborane initiators and initiate free radical formation. The radicals initiate polymerization and curing of the coating compositions. The most common decomplexing agents are acids, aldehydes, ketones, isocyanates, and anhydrides. The decomplexing agents can be used in gaseous form to initiate cure of coating compositions. Although effective in coating compositions on small scales, existing techniques employing such technology cannot be effectively scaled up and used in large scale OEM production facilities due to the cost and toxicity of the decomplexing agents. For example, many of the aforementioned decomplexing agents such as acetaldehyde, formaldehyde, isocyanates, maleic anhydride, methyl (ethyl) ketones, phthalic anhydride, and propionaldehyde are classified by the Environmental Protection Agency as hazardous air pollutants (HAPs). Accordingly, their use is severely restricted in large production facilities and is not environmentally friendly. Furthermore, many acids that are known for use as decomplexing agents are detrimental to properties of the cured coating and/or underlying substrates due to corrosive properties thereof, and the presence of the acids in the cured coating may be detrimental to the properties of the cured coating and/or the underlying substrates.
Accordingly, there remains an opportunity to develop an improved method of curing coating compositions that may be performed in the absence of external heating, UV light, peroxides, or azonitrile initiators and that can be used with complex shapes and sequestered surfaces. There also remains an opportunity to develop such a method that is environmentally friendly and useable in large production facilities with minimal emissions and pollution. There also remains an opportunity to develop such a method that reduces or eliminates the presence of acids in the cured coating that may be detrimental to the properties of the cured coating and/or underlying substrates.